Pediatric obesity is an increasingly prevalent public health crisis that contributes to the rising global burden of type 2 diabetes. Obesity and diabetes, at the most fundamental level, represent disorders of energy balance. In obesity, energy storage exceeds utilization. In diabetes, available energy sources (glucose) are improperly used. Energy balance is sensed and controlled by the cell's mitochondria. Focused study of key mitochondrial pathways that are disrupted in disorders of energy balance will improve our understanding of endocrine complications of obesity, and may lead to new treatment approaches. A particularly relevant group to study to better understand the intersection of mitochondrial dysfunction and obesity-related endocrine disorders are patients with primary (genetic) mitochondrial diseases, who develop similar profiles of disordered fat metabolism even in the absence of overt diabetes or obesity, including hypertriglyceridemia and low high density lipoprotein (HDL) cholesterol. Through the proposed K23 Mentored Patient-Oriented Research Career Development Award, I will investigate the mechanistic basis and metabolic consequences of disrupted lipid metabolism in primary mitochondrial disease. The proposed studies will directly support my main career goals as I progress towards independence as a physician-scientist. These goals are to [1] advance our understanding of the role of mitochondria, energy balance, and metabolism in pediatric endocrine disease, and to [2] apply physiologic insights to develop rational, targeted, and effective therapeutic interventions to improve the health of patients with endocrine and metabolic diseases. These goals will be advanced during the K23 award period through the pursuit of 3 overarching training and career development objectives: First, I will acquire didactic and technical knowledge critical for the successful design and execution of patient-oriented research in this area. During the three years of the proposed award, I will learn to perform valuable in vitro and in vivo phenotyping techniques under the guidance of content-area expert advisors. Specifically, in vitro utilization of stable isotopes and manipulation of cybrid cell lnes, and in vivo performance of hyperinsulinemic-euglycemic clamp studies complemented by stable isotope studies will be invaluable skills. Second, I will accrue new expertise in the diagnosis, evaluation, and management of patients with primary mitochondrial disease. This population has a large, unmet clinical need for better-informed subspecialists in all areas, particularly including Pediatric Endocrinology. They further provide a focused opportunity to study the role of the mitochondrial dysfunction in common endocrine disorders. This goal will be met by providing clinical care for mitochondrial disease patients and performing both in vitro and in vivo research investigations to characterize the extent and basis of their dyslipidemia. Third, I will transition to independence as an investigator. I will continue to work and develop productive multi-disciplinary collaborations with the expert members of my mentoring committee. I will gain necessary expertise to transition to independence through completing of the proposed studies, presenting at national meetings, publishing in peer-reviewed journals, and securing subsequent R01 funding.. Performing this proposed project will help accomplish these goals. These experiments will test our central hypotheses that: [1] altered cellular NAD+/NADH redox balance in the setting of primary respiratory chain (RC) impairment leads to increased de novo lipid synthesis from alpha-ketoglutarate (?KG)- derived citrate and decreased fatty acid oxidation (FAO) capacity and [2] the resulting excess accumulation of lipids in skeletal muscle causes skeletal muscle insulin resistance. To test these hypotheses, we will employ both in vitro and in vivo approaches. We will use in vitro human cell line models to test whether RC inhibition quantifiably increases de novo lipogenesis via reversed citric acid cycle flow. A complementary in vivo approach will look for evidence of increased de novo lipogenesis, muscle lipid accumulation, and decreased insulin sensitivity in (i) adults with primary RC disease, with hypertriglyceridemia but without DM. We will compare this group to (ii) appropriately matched healthy individuals. To assess whether reversed citric acid cycle flux might also contribute to hypertriglyceridemia in typical non-diabetic obese individuals, we will also study (iii) appropriately matched obese individuals. In future, we can extend aspects of these studies to the pediatric population. This project leverages many unique resources at CHOP and Penn. First, my primary mentor, Dr. Marni J. Falk, has a large and well-phenotyped cohort of patients with clear genetic diagnoses of mitochondrial disease that provides a ready source of ideal subjects, and tissues, in which to perform the proposed studies. CHOP's Center for Mitochondrial and Epigenomic Medicine (CMEM) further offers world-class infrastructure in this field. Penn's Diabetes Research Center (DRC) has faculty and core expertise already in place to assure successful completion of detailed metabolic phenotyping studies, where key faculty are comentors. Our collaborators at Penn's Center for Advanced Magnetic Resonance Imaging and Spectroscopy (CAMRIS) have developed metabolic imaging technologies to estimate and localize mitochondrial function. This work will allow me to establish a solid background from which to pursue future, independently-funded studies on the role of mitochondria, energy balance, and metabolism in pediatric endocrine disease